OO Design and Design Patterns in C++

OOP Design
Patterns in C++
Ganesh Samarthyam
ganesh@codeops.tech

“Applying design principles is the key to creating
high-quality software!”
Architectural principles:
Axis, symmetry, rhythm, datum, hierarchy, transformation

Why care about design quality and
design principles?

Poor software quality costs
more than $150 billion per year
in U.S. and greater than $500
billion per year worldwide
- Capers Jones

The city metaphor
Source: http://indiatransportportal.com/wp-content/uploads/2012/04/Trafﬁc-congestion1.jpg

The city metaphor
“Cities grow, cities evolve, cities
have parts that simply die while other
parts flourish; each city has to be
renewed in order to meet the needs of its
populace… Software-intensive systems
are like that. They grow, they evolve,
sometimes they wither away, and
sometimes they flourish…”
Grady Booch in the foreword for “Refactoring for Software Design Smells: Managing Technical Debt”, Girish Suryanarayana, Ganesh Samarthyam, Tushar Sharma, Morgan Kaufmann/Elsevier, 2014.

Modularisation in Java 9
Modularize JDK & JRE
Hide platform internal details such as sun.misc
Provide a module system for Java developers
Reference: http://paulbakker.io/java/java-9-modularity/

Example of beautiful design
int arr[] = {1, 4, 9, 16, 25}; // some values
// find the first occurrence of 9 in the array
int * arr_pos = find(arr, arr + 4, 9);
std::cout<< “array pos = “<< arr_pos - arr << endl;
vector<int> int_vec;
for(int i = 1; i <= 5; i++)
int_vec.push_back(i*i);
vector<int>::iterator vec_pos = find (int_vec.begin(), int_vec.end(), 9);
std::cout<< “vector pos = “<< (vec_pos - int_vec.begin());

For architects: design is the key!

What do we mean by “principles”?
“Design principles are key notions considered
fundamental to many different software design
approaches and concepts.”
- SWEBOK v3 (2014)

"The critical design tool for software development
is a mind well educated in design principles"
- Craig Larman

Robert C. Martin
Formulated many principles and described
many other important principles

Michael Feathers
Michael Feathers coined the acronym
SOLID in 2000s to remember ﬁrst ﬁve of the
numerous principles by Robert C. Martin

SOLID principles
S
Single Responsibility
Principle
Every object should have a single responsibility and
that should be encapsulated by the class
O Open Closed Principle
Software should be open for extension, but closed for
modification
L
Liskov’s Substitution
Principle
Any subclass should always be usable instead of its
parent class
I
Interface Segregation
Principle
Many client specific interfaces are better than one
general purpose interface
D
Dependency Inversion
Principle
Abstractions should not depend upon details. Details
should depend upon abstractions

3 principles behind patterns
Design'principles'
behind'pa0erns'
Program'to'an'
interface,'not'to'an'
implementa7on''
Favor'object'
composi7on''
over'inheritance'
Encapsulate'what'
varies'

Booch’s fundamental principles
Principles*
Abstrac/on*
Encapsula/on*
Modulariza/on*
Hierarchy*

How to apply principles in practice?
Design principles
Code
How to bridge
the gap?

Why care about refactoring?
As an evolving program is
continually changed, its
complexity, reflecting
deteriorating structure,
increases unless work is done
to maintain or reduce it
- Lehman's law of Increasing Complexity

What is refactoring?
Refactoring (noun): a change
made to the internal structure of
software to make it easier to
understand and cheaper to
modify without changing its
observable behavior
Refactor (verb): to restructure
software by applying a series
of refactorings without
changing its observable
behavior

What are smells?
“Smells'are'certain'structures'
in'the'code'that'suggest'
(some4mes'they'scream'for)'
the'possibility'of'refactoring.”''

Granularity of smells
Architectural+
+
Cyclic&dependencies&between&modules&
Monolithic&modules&&
Layering&viola9ons&(back&layer&call,&skip&layer&call,&ver9cal&layering,&
etc)&
Design+
+
God&class&
Refused&bequest&&
Cyclic&dependencies&between&classes&
Code+(implementa6on)++
+
Internal&duplica9on&(clones&within&a&class)&&
Large&method&&
Temporary&ﬁeld&&

What are design patterns?
recurrent solutions
to common
design problems
Pattern Name
Problem
Solution
Consequences

Why care about patterns?
❖ Patterns capture expert
knowledge in the form of
proven reusable solutions
❖ Better to reuse proven
solutions than to “re-invent”
the wheel
❖ When used correctly, patterns
positively inﬂuence software
quality
❖ Creates maintainable,
extensible, and reusable code

Design pattern catalog
Source: “Design Patterns: Elements of Reusable Object-Oriented Software”, Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, Addison-Wesley,1994

Design pattern catalog
Creational
Deals with controlled
object creation
Factory method, for
example
Structural
Deals with
composition of classes
or objects
Composite, for example
Behavioral
Deals with interaction
between objects/
classes and
distribution of
responsibility
Strategy, for example

An example
Source: “Refactoring for Software Design Smells: Managing Technical Debt”, Girish Suryanarayana, Ganesh Samarthyam, Tushar Sharma, Morgan Kaufmann/Elsevier, 2014

Object Oriented Design
- Best Practices

Design Practices: Even in C!
Consider the following methods from the C library:
void bcopy(const void *src, void *dst, size_t n);
void *memcpy(void *dst, const void *src, size_t n);
What is the problem in the interfaces of these methods? Does it follow the principles of good API design?

realloc - “does too many things!”

strtok - “stateful methods!” (not reentrant)

Intuitive class design?
class MyContainer {
private:
int m_size;
public:
void setLength(int size) { m_size= size; }
void setSize(int size) { m_size= size; }
int getLength() { return m_size; }
int getSize() { return m_size; }
// other members
}

Intuitive class design?
MyContainer * container = new Container;
container.setSize(0);
container.setLength(20);
cout << “Container's size is ” << container.getSize() <<endl;
cout << “Its length is “ container.getLength() <<endl;
// output:
// Container's size is 20
// Its length is 20

Calling virtual methods from actor
struct base {
base() {
vfun();
}
virtual void vfun() {
cout << “Inside base::vfunn”;
}
};
struct deri : base {
virtual void vfun() {
cout << “Inside deri::vfunn”;
}
};
int main(){
deri d;
}
// prints:
// Inside base::vfun

Calling virtual methods from actor
struct base {
base() {
base * bptr = this;
bptr->bar();
// even simpler ...
((base*)(this))->bar();
}
virtual void bar() =0;
};
struct deri: base {
void bar(){ }
};
int main() {
deri d;
}
// g++ output:
// pure virtual method called
// ABORT instruction (core dumped)

Temporary objects & NRV
class String {
public:
String(const char *str) : cstr(str) {}
String(const String& arg) {
std::cout<< “String cctor n”;
this->cstr = arg.cstr;
}
private:
const char * cstr;
}
String function(){
return String("Hello");
}
int main(){
String s = function();
}
// without NRV optimization on, this prints:
// String cctor
// with NRV optimization on, this prints:
// String cctor
// String cctor

Intuitive overloading?
Colour (int red, int green, int blue);
Colour (float hue, float saturation, float brightness);

static Colour* createRGBColour(int red, int blue, int green);
static Colour* createHSBColour(float hue, float saturation, float brightness);

void log(double val) {
// prints the natural logarithm value of val
}
void log(String str) {
// logs the string str into the log file
}

void log(double val) {
// prints the natural logarithm value of val
}
void log(String str) {
// logs the string str into the log file
}
void logarithm(double val);
void logger(String str);
// or
void log(double val);
void logger(String str);

Preventing inheritance
// C++ Example
class NoInherit {
private:
NoInherit() {
// code for constructor
}
public:
static NoInherit* createInstance() {
return new NoInherit();
}
};
// Creation of the object
NoInherit* nip = NoInherit::createInstance();

Beware of versioning problems
class Base { // provided by a third party tool
public:
virtual void vfoo(){ // introduced in version 2.0
cout<< ”vfoo in Base – introduced in a new version”;
}
// other members
};
class Derived : public Base { // some client code
public:
void vfoo(){ // was available in version 1.0
cout<< ”vfoo in Deri – now becomes overridden”;
}
// other members
};

Follow “safe” overriding
class Base {
public:
virtual void call(int val = 10)
{ cout << “The default value is :”<< endl; }
};
class Derived : public Base {
public:
virtual void call(int val = 20)
{ cout << “The default value is :”<< endl; }
};
// user code:
Base *b = new Derived;
b->call();
// prints
// The default value is: 10

Selectively introduce types
// in mymath.h
namespace math {
class scalar { /* … */ }
class vector { /* … */ }
// other members
}
// in use.cpp
// for using std::vector:
#include <vector>
using namespace std;
// for using the scalar class in your math namespace:
#include “mymath.h”
using namespace math;
int main() {
vector<scalar *> v;
// compiler error: vector is ambiguous
// does vector refer to std::vector or math::vector?
}

Selectively expose types to clients
namespace {
int someMem;
void somefunction();
};
// is a better way to limit names to file scope
// than the following:
static int someMem;
static void somefunction();

Hiding?
int x, y; // global variables x and y
class Point {
public:
int x, y; // class members x and y
Point(int x, int y); // function arguments x and y
};
// Point constructor for setting values of x and y data members
// C++
Point::Point(int x, int y) {
x = x;
y = y;
}

Beware of hiding
void foo { // outer block
int x, y;
{ // inner block
int x = 10, y = 20;
// hides the outer x and y
}
}

What’s that smell?
“Large class”
smell

Refactoring with “extract class”

Single Responsibility Principle
There should be only one
reason for a class to
change

Feature envy smell
Methods(that(are(more(interested(in(the(data(of(
other(classes(than(that(of(their(own(class(

Types of clones
• exactly(iden,cal(except'for'varia.ons'in'whitespace,'layout,'and'
comments'
Type'1'
• syntac,cally(iden,cal(except'for'varia.on'in'symbol'names,'whitespace,'
layout,'and'comments'
Type'2'
• iden.cal'except'some'statements(changed,(added,(or(removed(
Type'3'
• when'the'fragments'are'seman,cally(iden,cal(but'implemented'by'
syntac.c'variants'
Type'4'

Tools for clone detection
Simian
CPD PMD

Refactoring “incomplete library classes” smell
min/max' open/close' create/destroy' get/set'
read/write' print/scan' first/last' begin/end'
start/stop' lock/unlock' show/hide' up/down'
source/target' insert/delete' first/last' push/pull'
enable/disable' acquire/release' le:/right' on/off'

Open Closed Principle (OCP)
Bertrand Meyer
Software entities should be open for
extension, but closed for modiﬁcation

Variation Encapsulation Principle (VEP)
Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides
Encapsulate the concept that varies

Fundamental principle: Encapsulation
The principle of encapsulation advocates separation of concerns and
information hiding through techniques such as hiding implementation
details of abstractions and hiding variations

Enabling techniques for encapsulation

Design principles and enabling techniques

Liskov’s Substitution Principle (LSP)
It#should#be#possible#to#replace#
objects#of#supertype#with#
objects#of#subtypes#without#
altering#the#desired#behavior#of#
the#program#
Barbara#Liskov#

LSP violation
class Shape {
public:
// can throw any exceptions
virtual void rotate(int angle) = 0;
// other methods
};
class Circle : public Shape {
public:
virtual void rotate(int angle) throw (CannotRotateException) {
throw CannotRotateException();
}
// other methods
};
// client code
Shape *shapePtr = new Circle();
shapePtr->rotate(10);
// program aborts!

Private inheritance and LSP
class Base {};
class Derived : private Base {};
// user code
Base *b = new Derived();
// compiler error: conversion to inaccessible base class “Base”

Refused bequest smell
A"class"that"overrides"a"method"of"a"base"class"in"
such"a"way"that"the"contract"of"the"base"class"is"not"
honored"by"the"derived"class"

Refactoring
Replace inheritance
with delegation

Hands-on exercise
❖ Find out LSP violations in any code base
❖ Discuss how can it be ﬁxed.

Interface Segregation Principle (ISP)
Clients should not be forced to depend upon interfaces they do not use

Dependency Inversion Principle (DIP)
A. High level modules should not depend upon low level modules.
Both should depend upon abstractions.
B. Abstractions should not depend upon details. Details should depend
upon abstractions.

Suggested refactoring for this smell

Applying DIP in OO Design
Use references to interfaces/abstract classes as ﬁelds members, as
argument types and return types
Do not derive from concrete classes
Use creational patterns such as factory method and abstract factory
for instantiation
Do not have any references from base classes to its derived classes

3 principles behind patterns
Program to an interface, not to an
implementation
Favor object composition over inheritance
Encapsulate what varies

Cover key patterns through examples
It is not about the number of
patterns you know, but how
well you understand “why,
when, where, and how” to
apply them effectively

Scenario
enum ColorScheme { RGB, HSB, HLS, CMYK }
class Color {
private float red, green, blue; // for supporting RGB scheme
private float hue1, saturation1, brightness1; // for supporting HSB scheme
private float hue2, lightness2, saturation2; // for supporting HLS scheme
public Color(float arg1, float arg2, float arg3, ColorScheme cs) {
switch (cs) {
// initialize arg1, arg2, and arg3 based on ColorScheme value
}
}
}
• Assume that you need to support different Color schemes in your software
• RGB (Red, Green, Blue), HSB (Hue, Saturation, Brightness), and HLS (Hue,
Lightness, and Saturation) schemes
• Overloading constructors and differentiating them using enums can become
confusing
• What could be a better design?

A solution using Factory Method pattern
Color
+ GetRGBColor()
+ GetHSBColor()
+ GetHLSColor()
RGBColor
- red : float
- green : float
- blue : float
HSBColor
- hue : float
- saturation : float
- brightness : float
HLSColor
- hue : float
- lightness : float

A solution using Factory Method pattern
Color
+ GetColor(ColorType)
+ …
RGBColor
- red : float
- green : float
- blue : float
HSBColor
- hue : float
- brightness : float
HLSColor
- hue : float
- lightness : float

Factory method pattern: Structure

Factory method pattern: Discussion
❖ A class cannot anticipate the
class of objects it must create
❖ A class wants its subclasses to
specify the objects it creates
Deﬁne an interface for creating an object, but let subclasses decide which class to
instantiate. Factory method lets a class defer instantiation to subclasses.
❖ Delegate the responsibility
to one of the several helper
subclasses
❖ Also, localize the
knowledge of which
subclass is the delegate

Scenario
public:
MyLocale (std::string language, // e.g. “en" for English
std::string script, // e.g., “Arab” for Arabic
std::string country, // e.g., “us” for United States
std::string variant, // e.g., “TH” for Thai
std::string extensions) // e.g., “ca-buddhist” for Thai Buddhist Calendar
• Assume that you have a Locale class constructor that takes many “optional
arguments”
• Constraint: Only certain variants are allowed - you need to “disallow”
inappropriate combinations(e.g., invalid combination of country and variant) by
throwing IllformedLocaleException.
• Overloading constructors will result in “too many constructors”
• How will you design a solution for this?

Recommended solution
MyLocale aLocale =
new MyLocale.Builder
.setLanguage(“sr")
.setScript(“Latn")
.setRegion("RS")
.build();
• Create a Locale Builder that “builds” and returns an object step-by-step
• Validation will be performed by the individual set methods
• The build() method will return the “built” object

Builder pattern: structure

Builder pattern: Discussion
❖ Creating or assembling a
complex object can be tedious
Separate the construction of a complex object from its representation so that the same
construction process can create different representations.
❖ Make the algorithm for
creating a complex object
independent of parts that
make up the object and
how they are assembled
❖ The construction process
allows different
representations for the
object that is constructed

Real scenario #1
Initial design

Real scenario #1
❖ How will you refactor such
that:
❖ A speciﬁc DES, AES, TDES,
… can be “plugged” at
runtime?
❖ Reuse these algorithms in
new contexts?
❖ Easily add support for new
algorithms in Encryption?
Next change:
smelly design

Time to refactor!
Three strikes and you
refactor
Martin Fowler

Potential solution #1?
Broken
hierarchy!

Potential solution #2?
Algorithms not
reusable!

You’re right: Its Strategy pattern!

Strategy pattern: Discussion
❖ Useful when there is a set of
related algorithms and a client
object needs to be able to
dynamically pick and choose
an algorithm that suits its
current need
Deﬁnes a family of algorithms, encapsulates each one, and makes
them interchangeable. Strategy lets the algorithm vary
independently from clients that use it
❖ The implementation of each of the
algorithms is kept in a separate
class referred to as a strategy.
❖ An object that uses a Strategy
object is referred to as a context
object.
❖ Changing the behavior of a
Context object is a matter of
changing its Strategy object to the
one that implements the required
algorithm

Scenario
❖ Consider a Route class in an
application like Google Maps
❖ For ﬁnding shortest path from
source to destination, many
algorithms can be used
❖ The problem is that these
algorithms get embedded into
Route class and cannot be
reused easily (smell!)
Route
+ SetRoute(Location, Location)
+ FindShortestPathJohnson(): Path
+ FindShortestDijkstra(): Path
+ FindShortestBellmanFord(): Path
How will you refactor such that
a) Support for shortest path
algorithm can be added easily?
b) Separate path ﬁnding logic
from dealing with location
information.
- Source: Location
- Destination: Location

Hands-on exercise
❖ Try-out strategy pattern examples
❖ Discuss how overriding, functors (“function objects”),
and lambdas can be used for implementing strategy

How about this solution?
ShortestPathAlgos
+ FindPath(): Path
JohnsonsAlgo DijkstrasAlgo
Route
+ SetRoute(Location, Location)
+ ShortestPath(): Path
BellmanFordAlgo
- Source: Location
- Destination: Location

Real scenario #2
Initial design

Real scenario #2
How to add support for new
content types and/or algorithms?

Can you identify the pattern structure?

You’re right: Its Bridge pattern structure!

More design patterns to explore
Crea%onal)
• Abstract)factory))
• Builder))
• Factory)method))
• Prototype))
• Singleton)
Structural)
• Adapter))
• Bridge))
• Composite))
• Decorator))
• Facade))
• Flyweight))
• Proxy)
Behavioral)
• Chain)of)responsibility))
• Command))
• Interpreter))
• Iterator))
• Mediator))
• Memento))
• Observer))
• State))
• Strategy))
• Template)method))
• Visitor)
deals with object
creation
deals with composition
of classes or objects
deals with interaction
between objects/
classes and
distribution of
responsibility

Hands-on exercise
❖ Try-out bridge pattern sample source code

Scenario
Report
+ Open()
+ Populate()
+ Print()
+ Save()
SummaryHTMLReport SummaryRichReport DetailedHTMLReport DetailedRichReport

FileFormat
+ Save()
HTMLFormat RichTextFormat
Report
+ Open()
+ Save(ReportType)
+ …
SummaryReport DetailedReport

You’re right: Its Bridge pattern!

An example of Bridge pattern

Bridge pattern: Discussion
❖ An abstraction can be designed as an
interface with one or more concrete
implementers.
❖ When subclassing the hierarchy, it
could lead to an exponential number of
subclasses.
❖ And since both the interface and its
implementation are closely tied
together, they cannot be independently
varied without affecting each other
Decouples an abstraction from its implementation so
that the two can vary independently
❖ Put both the interfaces and the
implementations into separate class
hierarchies.
❖ The Abstraction maintains an object
reference of the Implementer type.
❖ A client application can choose a desired
abstraction type from the Abstraction
class hierarchy.
❖ The abstraction object can then be
conﬁgured with an instance of an
appropriate implementer from the
Implementer class hierarchy

Scenario
Initial design
TextView
+ Draw()
BorderedTextView
+ Draw()
+ DrawBorder()

Scenario

Supporting new requirements
Revised design with
new requirements
TextView
+ Draw()
BorderedTextView
+ Draw()
+ DrawBorder()
ScrollableTextView
ScrollableBordered
TextView
- borderWidth
+ Draw()
+ ScrollTo()
- ScrollPosition
+ Draw()
+ ScrollTo()
+ DrawBorder()
- ScrollPosition
- borderWidth

Scenario
❖ How will you refactor such
that:
❖ You don't have to “multiply-
out” sub-types? (i.e., avoid
“explosion of classes”)
❖ Add or remove
responsibilities (e.g.,
scrolling) at runtime?
Next change:
smelly design

VisualComponent
+ Draw()
TextView
+ Draw()
ScrollDecortor BorderDecorator
+ Draw()
+ ScrollTo()
- ScrollPosition
+ Draw()
+ DrawBorder()
- borderWidth
Decorator
+ Draw() component->Draw()
Decorator::Draw()
DrawBorder()
Decorator::Draw()
ScrollTo()

At runtime (object diagram)

Can you identify the pattern?
VisualComponent
+ Draw()
TextView
+ Draw()
ScrollDecortor BorderDecorator
+ Draw()
+ ScrollTo()
- ScrollPosition
+ Draw()
+ DrawBorder()
- borderWidth
Decorator
+ Draw() component->Draw()
Decorator::Draw()
DrawBorder()
Decorator::Draw()
ScrollTo()

You’re right: Its Decorator pattern!

Decorator pattern: Discussion
❖ Want to add responsibilities to
individual objects (not an
entire class)
❖ One way is to use inheritance
❖ Inﬂexible; static choice
❖ Hard to add and remove
responsibilities dynamically
Attach additional responsibilities to an object dynamically. Decorators
provide a ﬂexible alternative to subclassing for extending functionality
❖ Add responsibilities through
decoration
❖ in a way transparent to the
clients
❖ Decorator forwards the requests to
the contained component to
perform additional actions
❖ Can nest recursively
❖ Can add an unlimited number
of responsibilities dynamically

Hands-on exercise
❖ Try-out decorator pattern sample source code

Scenario
a) How do we treat ﬁles
and folders alike?
b) How can we handle
shortcuts?
File
+ GetName()
+ GetSize()
+ …
- name: String
- size: int
- type: FileType
- data: char[]
- …
Folder
+ GetName()
+ GetFiles()
+ GetFolders()
+ …
- name: String
- ﬁles[]: File
- folders[]: Folder

FileItem
+ GetName()
+ GetSize()
+ Add(FileItem)
+ Remove(FileItem)
Folder
+ GetFiles()
+ …
File
- ﬁles: FileItem
+ GetType()
+ GetContents()
+ …
- type: FileType
- data: char[]
Shortcut
+ GetLinkedFileItem()
+ …
- linkToFile: FileItem

Composite pattern

Composite pattern: Discussion
❖ There are many situations where
a group of components form
larger components
❖ Simplistic approach: Make
container component contain
primitive ones
❖ Problem: Code has to treat
container and primitive
components differently
Compose objects into tree structures to represent part-whole hierarchies. Composite
lets client treat individual objects and compositions of objects uniformly.
❖ Perform recursive
composition of
components
❖ Clients don’t have to
treat container and
primitive components
differently

Hands-on exercise
❖ Try-out bridge decorator sample source code

Decorator vs. Composite
Decorator and composite structure looks similar:
Decorator is a degenerate form of Composite!
Decorator Composite
At max. one component Can have many components
Adds responsibilities Aggregates objects
Does not make sense to have
methods such as Add(),
Remove(), GetChid() etc.
Has methods such as Add(),
Remove(), GetChild(), etc.

Scenario
How will you design to share the
characters to save space?

Flyweight as a solution

Flyweight pattern: structure

Flyweight pattern: Discussion
❖ When an application uses a
large number of small objects,
it can be expensive
❖ How to share objects at
granular level without
prohibitive cost?
Use sharing to support large numbers of ﬁne-grained objects efﬁciently
❖ When it is possible to share
objects (i.e., objects don't
depend on identity)
❖ When object’s value
remain the same
irrespective of the contexts
- they can be shared
❖ Share the commonly used
objects in a pool

Scenario
Source: http://www.javaworld.com/article/2074068/learn-java/take-control-with-the-proxy-design-pattern.html
How to load objects like
large images efﬁciently “on-
demand?”

Proxy pattern: example

Proxy pattern: structure

Proxy pattern: Discussion
❖ How to defect the cost of full
creation and initialization until
we really need it?
Provide a surrogate or placeholder for another object to control access to it.
❖ When it is possible to share
objects
❖ When object’s value
remain the same
irrespective of the
contexts - they can be
shared
❖ Share the commonly used
objects in a pool

Scenario
In an application similar to MS Paint, assume that a class (say
ShapeArchiver) is responsible for archiving information about all
the drawn shapes. Similarly, another class (say Canvas) is
responsible for displaying all drawn shapes. Whenever any
change in shapes takes place, you need to inform these two
classes as to the changed information.
So, how you would like to implement this notiﬁcation?

Scenario
How to exploit:
a) common code segments?
b) provide extensibility for
supporting other kinds of
compressed files (e.g., rar)?
• You have code segments and steps for creating different kinds of compressed files,
such as zip file and gzip files. How will you unify the common steps for
compressing files and support new compression formats easily?

Refactoring to Template Method
CompressTextFile
+ getFileName(String): String
+ writeFile(String): OutputStream
+ closeFile(OutputStream): void
ZipTextFile
GZIPTextFile

Template Method: Structure
Source: https://www.safaribooksonline.com/library/view/swift-2-design/9781785887611/graphics/4582_05_07.jpg

Template Method Pattern: Discussion
Deﬁne the skeleton of an algorithm in an operation, deferring some
steps to subclasses. Template Method lets subclasses redeﬁne certain steps
of an algorithm without changing the algorithm's structure.

Template Method in CppUnit
https://github.com/hvoigt/cppunit/blob/master/src/cppunit/TestCase.cpp

Scenario
How to separate:
a) code generation logic
from node types?
b) how to support different
target types?
class Plus extends Expr {
private Expr left, right;
public Plus(Expr arg1, Expr arg2) {
left = arg1;
right = arg2;
}
public void genCode() {
left.genCode();
right.genCode();
if(t == Target.JVM) {
System.out.println("iadd");
}
else { // DOTNET
System.out.println("add");
}
}
}

Group exercise -
Understanding visitor pattern

A solution using Visitor pattern
class Plus extends Expr {
private Expr left, right;
public Plus(Expr arg1, Expr arg2) {
left = arg1;
right = arg2;
}
public Expr getLeft() {
return left;
}
public Expr getRight() {
return right;
}
public void accept(Visitor v) {
v.visit(this);
}
}
class DOTNETVisitor extends Visitor {
public void visit(Constant arg) {
System.out.println("ldarg " + arg.getVal());
}
public void visit(Plus plus) {
genCode(plus.getLeft());
genCode(plus.getRight());
System.out.println("add");
}
public void visit(Sub sub) {
genCode(sub.getLeft());
genCode(sub.getRight());
System.out.println("sub");
}
public void genCode(Expr expr) {
expr.accept(this);
}
}

Visitor pattern: structure

Visitor pattern: call sequence

Visitor pattern: Discussion
❖ Many distinct and unrelated
operations need to be
performed on objects in an
object structure, and you want
to avoid “polluting” their
classes with these operations
Represent an operation to be performed on the elements of an object structure.
Visitor lets you deﬁne a new operation without changing the classes of the
elements on which it operations
❖ Create two class
hierarchies:
❖ One for the elements
being operated on
❖ One for the visitors that
deﬁne operations on the
elements

Patterns discussed so far
✓ Builder pattern
✓ Factory method pattern
✓ Strategy pattern
✓ Bridge pattern
✓ Decorator pattern
✓ Observer pattern
✓ Composite pattern
✓ Flyweight pattern
✓ Proxy pattern
✓ Visitor pattern
✓ Template method pattern
✓ Interpreter pattern

Many other patterns other than GoF
template <typename T>
class MyClass : public Counter<MyClass<T>> { };

Hands-on exercise
❖ Try-out CRTP sample source code

Design Patterns
- A Case Study

Designing a document editor
• How to maintain a document structure?
• How to support formatting?
• How to support embellishments
(highlighting, marking, etc) with ease?
• Support multiple look-and-feel
standards
• Support multiple windowing systems
(mac, windows, motif, etc)
• How to support user operations and
handle user events?
• How to support spellchecking and
hyphenation?
• ….

“Recursive composition”

Solution using Composite pattern

More “recursive composition”

“Recursive composition” - class design

Supporting bordering, scrolling, …

Supporting multiple look-and-feel

Abstract factory: Structure

Separating implementation dependencies

Supporting user operations

Command pattern: Structure

Supporting undo/redo: Command history

Traversing document (e.g., spell check)

Iterator pattern: Structure

Iterator pattern: Another example

Patterns discussed in this case-study
✓ Composite pattern
✓ Decorator pattern
✓ Abstract factory pattern
✓ Bridge pattern
✓ Command pattern
✓ Iterator pattern

Exercise: Create a suitable design
Note:&Responsibility&of&
rendering&the&line&in&chosen&
style&is&with&underlying&OS&/&
pla;orm&

Suggested refactoring for this smell

Beware of “patterns mania”!

Image/video credits
❖ http://en.wikipedia.org/wiki/Fear_of_missing_out
❖ http://lesliejanemoran.blogspot.in/2010_05_01_archive.html
❖ http://javra.eu/wp-content/uploads/2013/07/angry_laptop2.jpg
❖ https://www.youtube.com/watch?v=5R8XHrfJkeg
❖ http://womenworld.org/image/052013/31/113745161.jpg
❖ http://www.fantom-xp.com/wallpapers/33/I'm_not_sure.jpg
❖ https://www.flickr.com/photos/31457017@N00/453784086
❖ https://www.gradtouch.com/uploads/images/question3.jpg
❖ http://gurujohn.files.wordpress.com/2008/06/bookcover0001.jpg
❖ http://upload.wikimedia.org/wikipedia/commons/d/d5/Martin_Fowler_-_Swipe_Conference_2012.jpg
❖ http://www.codeproject.com/KB/architecture/csdespat_2/dpcs_br.gif
❖ http://upload.wikimedia.org/wikipedia/commons/thumb/2/28/Bertrand_Meyer_IMG_2481.jpg/440px-
Bertrand_Meyer_IMG_2481.jpg
❖ http://takeji-soft.up.n.seesaa.net/takeji-soft/image/GOF-OOPLSA-94-Color-75.jpg?d=a0
❖ https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/OOP_ObjC/Art/watchcalls_35.gif
❖ http://www.pluspack.com/files/billeder/Newsletter/25/takeaway_bag.png
❖ http://cdn1.tnwcdn.com/wp-content/blogs.dir/1/files/2013/03/design.jpg
❖ http://img01.deviantart.net/d8ab/i/2016/092/c/2/may_the_force_be_with_you___yoda_flag_by_osflag-d9xe904.jpg

ganesh@codeops.tech @GSamarthyam
www.codeops.tech slideshare.net/sgganesh
+91 98801 64463 bit.ly/sgganesh

OO Design and Design Patterns in C++

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